1. Field of the Invention
The present invention relates to a battery pack. More specifically, the present invention relates to a battery pack having, as single cells, a plurality of secondary cells connected to one another. This application claims priority to Japanese Patent Application No. 2016-182413, filed on Sep. 16, 2016, the entire contents of which are hereby incorporated by reference into the present application.
2. Description of the Related Art
Battery packs in which, as single cells, a plurality of secondary cells such as lithium-ion secondary cells and nickel-hydrogen cells or power storage elements such as capacitors are connected in series or in parallel by bus bars, have become increasingly important as the in-car power sources or the power sources for personal computers, portable terminals and the like. Particularly, due to their light weights and high-energy densities, the battery packs having lithium-ion secondary cells as single cells have favorably been used in the in-car high-output power sources and the like.
The lithium-ion secondary cells used as the single cells are each generally configured by storing an electrode body with sheet-shaped positive and negative electrodes in a battery case. In recent years, in order to improve battery performance, there has been developed the lithium-ion secondary cell with a layered electrode body in which rectangular positive and negative electrode sheets are layered.
A battery pack having a plurality of such lithium-ion secondary cells connected to one another is required to provide a high level of safety, not to mention excellent battery performance. For example, in some cases when single cells produce heat during use, the single cells add heat to one another, increasing the temperature of the entire battery pack. In this case, materials inside the single cells become denatured by heat, possibly ruining the physical or chemical stability thereof.
In order to keep the stability by preventing the increase in the temperatures of the cells, countermeasures such as the ones described in Japanese Patent Application Laid-open No. 2003-31187 and Japanese Patent Application Laid-open No. 2009-266402 have been proposed. Japanese Patent Application Laid-open No. 2003-31187, for example, describes a secondary cell and a battery pack with a plurality of the secondary cells in which an endothermic agent composed of a compound capable of conducting an endothermic reaction is added to an envelope film or an enveloping pack material of the secondary cell and the battery pack, in order to inhibit a temperature increase that can occur upon abnormal heat production of the cells.